1. Field of the Invention
The present invention relates to a method and device for detecting a liquid level, and more specifically, concerns a liquid-level detection method and device, in which a nozzle is lowered toward a liquid level while a gas is being suctioned or discharged from a tip of the nozzle through a pipe by a pump so that, when the inner pressure of the pipe changes, it is judged that the tip of the nozzle has come into contact with the liquid level.
Such liquid-level detection method and device are applied to a biochemical automatic analyzer for measuring a sample such as blood or urine.
2. Description of the Prior Art
Liquid-level detection methods of this type have been disclosed in, for example, Japanese Unexamined Patent Application Nos. 196964/1990 (Hei 2-196964) and 243960/1990 (Hei 2-243960) gazettes.
The liquid-level detection method disclosed in the above-mentioned Japanese Unexamined Patent Application No. 196964/1990 (Hei 2-196964) is provided with the steps of preliminarily suctioning air into a nozzle chip; shifting the chip toward a position above a sample tube; lowering the chip toward a liquid level inside the sample tube while discharging air that has been suctioned; setting an inner pressure of an air hose (pipe) that has been obtained by a sampling process after a lapse of a predetermined period of time from the start of the lowering of the chip as a reference value; detecting a change in the inner pressure from the reference value that takes place when a suction inlet of the chip has come into contact with the liquid level and is closed; and making judgment that a tip of the nozzle has reached the liquid level when the change has been detected.
The liquid-level detection method disclosed by the above-mentioned Japanese Unexamined Patent Application No. 243960/1990 (Hei 2-243960) is provided with a pressure sensor for detecting a pressure of a flowing system (corresponding to the pipe) that allows a pump and a pipet (corresponding to the chip) to communicate with each other, and detects the liquid level from the pressure change inside the flowing system detected by the pressure sensor while the pipet is lowered with the pump being driven.
In these liquid-level detection methods, when the tip of the chip has come into contact with the liquid level, the air discharge from the tip of the chip is blocked by the liquid level with the result that the inner pressure of the pipe is raised; thus, it is judged that the tip position of the chip at this time corresponds to the liquid level position.
For example, in an automatic analyzer for measuring a sample such as blood or urine, it has been conventionally known that in an automatic injection, a slight difference in the amount of injection of a sample is greatly reflected in the measured value. Therefore, the depth of insertion of the chip is minimized by detecting the liquid level of a sample so that it is possible to reduce adhesion of the sample to the outer wall of the chip, and consequently to improve the precision in the amount of injection.
For example, in the case when the sample is a liquid such as blood and urine, bubbles and films can exist above the sample liquid level inside a container in which the sample is housed. When a liquid-level detecting process is carried out with such bubbles and films above the liquid level, the inner pressure of the pipe increases when the tip of the chip has come into contact with the bubbles and films, resulting in the control part making a judgment that the position of the tip of the chip at this time corresponds to the liquid level position, and starts the suction process, failing to carry out an accurate suction process for the sample.
The objective of the present invention is to provide a liquid-level detection method and device, which can carry out an accurate liquid level detection even when bubbles and films are located above the liquid level.
The liquid-level detection method in accordance with the present invention is provided with: a lowering step of lowering a nozzle toward a liquid level while a gas is being suctioned or discharged from the tip of the nozzle by a pump; a detection step of, upon detection of a change in the inner pressure of a pipe connected to the nozzle, stopping the suction process or discharging process of the pump and the lowering process of the nozzle; and a judging step of making a judgment that the tip of the nozzle has come into contact with the liquid level when a predetermined time has elapsed with the inner pressure in the pipe being maintained within a predetermined permissible range of the amount of change.
Here the term xe2x80x9cnozzlexe2x80x9d is used under the assumption that the nozzle includes the chip, in the case when a chip is attached to the tip of a nozzle.
The liquid-level detection device in accordance with the present invention is provided with: a pump for suctioning and discharging a gas so that a liquid is suctioned and discharged from a tip of a nozzle; a nozzle driving mechanism for raising and lowering the nozzle; a pressure sensor for measuring an inner pressure of a pipe connected to the nozzle; and a control part which controls the pump and the nozzle driving mechanism so as to lower the nozzle toward the liquid level while suctioning or discharging the gas from the tip of the nozzle, and to stop the operations of the pump and the nozzle driving mechanism upon detection of a change in the inner pressure of the pipe by the pressure sensor, and makes a judgment that the tip of the nozzle has come into contact with the liquid level when a predetermined time has elapsed with the output of the pressure sensor being maintained within a predetermined permissible range of the amount of change.
In the liquid-level detection method and device in accordance with the present invention, when the inner pressure of the pipe connected to the nozzle changes, the suction process or discharging process of the pump and the lowering operation of the nozzle are discontinued, and a judgment is made as to whether or not a predetermined time has elapsed with the inner pressure in the pipe being maintained within a predetermined permissible range of the amount of change. When the change in the inner pressure of the pipe is caused by bubbles and films located above the liquid level, the pressure change is a temporary phenomenon, and when the suction process or discharging process of the gas from the tip of the nozzle is discontinued, the inner pressure of the pipe varies to approach the pressure open to atmosphere as time elapses. In contrast, in the case when a change in the inner pressure of the pipe is caused by the liquid level, the inner pressure of the pipe barely changes even when time has elapsed after the stoppage of the suction or discharge of the gas from the nozzle tip.
In this manner, in the liquid-level detection method and device in accordance with the present invention, upon detection of a change in the inner pressure of the pipe, the suction or discharging process of the pump and the lowering operation of the nozzle is discontinued, and the following pressure variations are monitored so that it is judged whether the change in the inner pressure of the pipe is caused by the liquid level or bubbles and films; thus, it is possible to reduce the probability of misdetection due to bubbles and films, consequently to accurately detect the liquid level.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.